The present invention relates to an acceleration sensor for use in a motor vehicle, an aircraft, a household electrical appliance or the like.
FIGS. 19 and 20 are a side sectional view and a front elevational view of a conventional acceleration sensor, respectively. The conventional acceleration sensor includes a sensor chip 101 for converting into an electrical signal an acceleration applied in a direction of a sensitive axis X parallel to a board face BF of a printed circuit board 110, an IC chip 103 acting as a processing circuit for processing the electrical signal of the sensor chip 101, an oblique spacer 102 for supporting the sensor chip 101 and a package 104 for accommodating the IC chip 103 and the oblique spacer 102 having the sensor chip 101 mounted thereon. The package 104 is of a shape of a rectangular box and has at its underside a mounting face MF to be mounted on the board face BF of the printed circuit board 110. The mounting face MF of the package 104 is provided on one of outer faces of the rectangular box, which has a minimum area in the outer faces. A plurality of terminals 106 for transmitting signals between outside and the sensor chip 101 and the IC chip 103 are provided at a lower portion of each of front and rear faces of the package 104 so as to be secured to the board face BF of the printed circuit board 110 by solder 111 such that the mounting face MF of the package 104 is mounted on the board face BF of the printed circuit board 110.
Meanwhile, an opening formed on the front face of the package 104 is covered by a cap 105. Pads 112 of the sensor chip 101 and pads 114 of the IC chip 103 are, respectively, connected by bonding wires 109 to pads 113 formed on an inner face of the package 104 and pads 115 formed on the inner face of the package 104. Furthermore, a wiring pattern 107 is provided for connecting the pads 113 and 115 and the terminals 106.
As shown in FIGS. 21 and 22, the conventional sensor chip 101 has a so-called cantilever construction including a first substrate 120 formed by a semiconductor substrate and a second substrate 130 joined to a rear face of the first substrate 120. As best shown in FIG. 23, the first substrate 120 is formed with a weight portion 123, a deflective portion 122 which is formed on a principal surface of the first substrate 120 and has one end coupled integrally with the weight portion 123, a support portion 121 for pivotally supporting the weight portion 123 via the deflective portion 122, which is integrally coupled with the other end of the deflective portion 122 and a piezoresistance portion 124 disposed at the deflective portion 122. The piezoresistance portion 124 acts as a sensing element for detecting a deformation of the deflective portion 122 so as to convert a degree of the deformation of the deflective portion 122 into an electrical signal representing an acceleration. Meanwhile, a reference numeral xe2x80x9c125xe2x80x9d in FIGS. 21 and 22 denotes a stopper. By deflection of the deflective portion 122 upon application of an acceleration to the sensor chip 101, the piezoresistance portion 124 is also deflected so as to change its resistance value, so that an electrical signal corresponding to the resistance value is outputted as the acceleration.
As shown in FIG. 23, the sensor chip 101 of the cantilever construction should be inclined at an angle xcex8 relative to a perpendicular on the board face BF of the printed circuit board 110 such that a straight line connecting a fulcrum B of deflection of the deflective portion 122 and a center C of gravity of the weight portion 123 is perpendicular to the sensitive axis X, i.e., a direction of application of the acceleration. If this angle xcex8 is not correct, the deflective portion 122 is deflected even when the acceleration is 0 G, so that off-axis sensitivity of the acceleration sensor increases, thereby resulting in an erroneous output of the acceleration sensor. The oblique spacer 102 supports the sensor chip 101 so as to define this angle xcex8 of inclination of the sensor chip 101.
Meanwhile, the sensor chip 101 may also have a so-called fixed beam construction in which opposite ends of the weight portion 123 are, respectively, fixed by a pair of the support portions 121 through a pair of the deflective portions 122. In this case, since the angle xcex8 in FIG. 23 is zero, the principal surface of the sensor chip 101 is perpendicular to the sensitive axis Z.
In the conventional acceleration sensor referred to above, the sensitive axis X is substantially perpendicular to the principal surface of the sensor chip 101. Hence, in case an acceleration parallel to the board face BF of the printed circuit board 110 is detected by the conventional acceleration sensor, the mounting face MF of the package 104 having a shape of a rectangular box should be provided on one of outer faces of the rectangular box, which has a minimum area in the outer faces, so that undesirable inclination of the package 104 due to its inaccurate mounting is likely to become large and thus, such a disadvantage may be incurred that off-axis sensitivity of the acceleration sensor becomes large due to inaccurate parallelism between the sensitive axis X and the board face BF of the printed circuit board 110.
Meanwhile, as shown in FIG. 19, the package 104 is formed by a multi-layer ceramic package in which ceramic plates 104a to 104e are laminated on each other such that lamination faces of the ceramic plates 104a to 104e are perpendicular to the board face BF of the printed circuit board 110. Since a ceramic sheet is split into the ceramic plates 104a to 104e by using a break method which is an inexpensive method for splitting the ceramic sheet into a plurality of ceramic plates each having a desired size, burrs are formed on end faces and side faces of the package 104, which are split faces of the ceramic plates 104a to 104e, so that it is difficult to obtain accurate flatness of the package 104 and thus, undesirable inclination of the package 104 due to its inaccurate mounting is apt to become large, thereby resulting in such a problem that off-axis sensitivity of the acceleration sensor becomes large.
Furthermore, in case the package 104 is formed by the multi-layer ceramic package, a planar conductive pattern having such a size as to enable wire bonding can be formed only on faces of the package 104 parallel to the lamination faces of the ceramic plates 104a to 104e. Here, if the sensor chip 101 has a cantilever construction in which the principal surface, i.e., a wire bonding face of the sensor chip 101 is not perpendicular to the sensitive axis X as described above, the wire bonding face for the pads 112 in the sensor chip 101 and a wire bonding face for the pads 113 in the package 104 are not parallel to each other, so that wire bonding of the pads 112 and 113 should be performed between the wire bonding faces of the sensor chip 101 and the package 104, which are not parallel to each other and thus, such an inconvenience is encountered that it is difficult to secure reliability of wire bonding of the pads 112 and 113.
The present invention has for its object to provide, with a view to eliminating the above mentioned drawbacks of prior art, an acceleration sensor in which by lessening undesirable inclination of a sensor chip due to its inaccurate mounting, off-axis sensitivity of the acceleration sensor is reduced and reliability of wire bonding portions is secured.
In order to accomplish this object of the present invention, an acceleration sensor according to the present invention includes a sensor chip for converting into an electrical signal an acceleration applied in a direction of a sensitive axis parallel to a board face of a printed circuit board, which has a first wire bonding face. A wiring base supports the sensor chip and includes second and third wire bonding faces and a wiring pattern formed on the second and third wire bonding faces. A package accommodates the wiring base having the sensor chip mounted thereon and has a mounting face to be mounted on the board face of the printed circuit board and a fourth wire bonding face substantially parallel to the mounting face. The wiring base is mounted on the package such that a difference between heights of the third and fourth wire bonding faces from the board face of the printed circuit board and an angle formed between the third and fourth wire bonding faces are set to small values enabling the wiring pattern on the third wire bonding face of the wiring base to be connected to pads on the fourth wire bonding face of the package. The sensor chip is mounted on the wiring base such that the wiring pattern on the second wire bonding face of the wiring base can be connected to pads on the first wire bonding face of the sensor chip.